Oligomeric materials containing primary amine groups are desirable in numerous applications, particularly coatings, adhesives and curatives for epoxy, isocyanate, aminoplast and other reactive systems. Preparation of low molecular weight or oligomeric primary amine containing polymers has previously been impossible even using the desired precursor monomer N-vinylformamide. Even with use of large amounts of chain transfer agents suitably low molecular weights could not be achieved.
Polymerization of N-vinylformamide (NVF) has been widely reported under free radical conditions to give high molecular weight homo and copolymers. Polymers containing mixtures of primary, secondary and tertiary amine groups (e.g., polyethyleneimine) or oxidatively unstable ethyleneamines (mainly secondary, with terminal primary amines) have been prepared by other routes and used previously.
Vinylamine polymers with molecular weights (Mw) from 3,000 to 6.7 million have been reported. U.S. Pat. No. 4,018,826 and corresponding U.S. Pat. No. Reissue Re.30,362 disclose the preparation of poly(vinylamine) salts of mineral acids by polymerizing vinylacetamide with a free radical polymerization catalyst and hydrolyzing the polymer to the desired amine salts by contacting the p-vinylacetamide with an aqueous solution of the corresponding mineral acid. Poly(vinylamine) product of about 3,000 to about 700,000 molecular weight (4000 to about 1,000,000 for the salt product) is suggested. U.S. Pat. No. 4,808,683 discloses making high molecular weight N-vinylformamide copolymer by free radical polymerization.
Zurakowska-Orszagh, et al., J. Polym. Sci, Polym Chem Ed., 19, 1981, 363-369, describes the synthesis of stable complexes of Lewis acids with acrylamide monomers. These complexes have monomer/Lewis acid molar ratios of 2 to 4, and therefore are basically stoichiometric complexes. These complexes are not polymerized, nor do they polymerize spontaneously. Zurakowska-Orszagh, Mirowski and Chajewski, J. Polym. Sci, Polym Chem Ed., 20, 1982, 1775-84 describes the polymerization of acrylamide monomers in the presence of zinc halide Lewis acids. These polymerizations are not ionic, but clearly initiated by radiation induced free radical processes. The authors specifically cite inhibition by added oxygen, a known free radical inhibitor, as proof of the free radical mechanism.
U.S. Pat. No. 4,952,656 discloses polymerization of NVF to extremely high molecular weight polymers above 3 MM using inverse emulsion conditions. NVF polymers and copolymers have the unusual ability to be hydrolyzed easily under acid or base conditions to the corresponding polyvinylamine, giving polymers with highly reactive primary amine groups or, when protonated, high levels of cationic charge. Such materials have numerous applications as curatives in coatings and adhesives, for papermaking, water treatment, enhance oil recovery, for personal care polymers, in biomedical applications and in mineral recovery.
To prepare free radical polymers, it is well known that oxygen and other radical inhibitors must be removed in order to allow the radical chains to grow, consuming the monomer, without excessive use of expensive initiators. The resulting polymers are also rather high in molecular weight, 600,000 to 2 million under bulk polymerization techniques for vinylamide monomers. Such polymers are highly viscous and difficult to process, even in solution (typically in water, one of the few good solvents for poly-N-vinylformamide (PNVF) or poly-vinylamine (PVAm)). For many applications it is desired to have very low molecular weight polymer, e.g., to give low viscosities or better miscibility and solubility, or in reactive systems to give moderate crosslink densities that prevent the system from becoming too brittle. This is typically done with free radical polymerizations with the addition of chain transfer agents, such as dodecanethiol, bromotrichloromethane, or isopropanol. Such agents add unwanted cost and co- or by-products (such as halogens) to the polymer and result in unwanted high polydispersity or too high molecular weights. Unreacted chain transfer agents are also difficult to remove from the mixture.
Alternatively, the free radical polymerization may be run by delay feeding monomer and/or at high dilution in a solvent. This option gives poor kettle utilization, requires the use of much solvent which may be expensive, toxic or hard to remove and, with NVF, still gives polymers with molecular weights in the 50,000 range at reasonable (10%) monomer levels. Running the polymerization at high temperature with excess initiator, particularly with the delay feeds as described above, is not suitable because of the low thermal stability of NVF in the condensed phase and its tendency to auto decompose to colored byproducts. The added initiator is also a significant cost factor.
In addition to free radical polymerization, amine and formamide containing polymers have been made via other means, such as the condensation of ethylene glycol and ammonia, ethanolamine or ethylenediamine or combinations of these compounds at high temperatures in the presence of a catalyst to give polyethylenepolyamines. Such materials give complex mixtures with mainly secondary and tertiary amines and numerous cyclic, substituted piperazine structures. Such materials have limited thermal and oxidative stability and frequently have color problems. Similar materials can be made using ethylenedichloride and ammonia or ethylenediamine, along with a base. These materials produce significant salt byproducts and may also give some quaternary nitrogens.
Polymerization of aziridine produces highly branched polyamines with nearly equal quantities of primary, secondary and tertiary nitrogens. In addition to the problems cited for condensation reactions, the monomer is highly toxic, carcinogenic and explosive.
German Patent Application DE 4030-380-A discloses an anionic N-vinylformamide polymerization process using a basic catalyst to form primarily an NVF dimer and an oligomer with a hydrolytically unstable nitrogen containing backbone.
M. Sawamoto, TRIP 1 No 4, April 1993, 111-115 discloses cationic polymerization of vinyl monomers using an initiator such as a protonic acid or metal halide. Hashimoto, et al., Poly. Sci., 26, 3361-3374 (1988) disclose cationic polymerization of functional vinyl ethers with pendant primary amino groups in which phthalimide is used as a protecting group for the amino pendant. The polymerization is initiated using a hydrogen iodide/iodine (HI/I.sub.2) system.